Microchannel plates are used as electron multipliers in image intensifiers. They are thin glass plates having an array of channels extending there through and are located between a photocathode and a phosphor screen. An incoming electron from the photocathode enters the input side of the microchannel plate and strikes a channel wall. When voltage is applied across the microchannel plate, these incoming or primary electrons are amplified, generating secondary electrons. The secondary electrons then exit the channel at the back end of the micrcochannel plate and are used to generate an image on the phosphor screen.
In general, fabrication of a microchannel plate starts with a fiber drawing process, as disclosed in U.S. Pat. No. 4,912,314, issued Mar. 27, 1990 to Ronald Sink, which is incorporated herein by reference in its entirety. For convenience, FIGS. 1–4, disclosed in U.S. Pat. No. 4,912,314, are included herein and discussed below.
In FIG. 1 there is shown a starting fiber 10 for the microchannel plate. Fiber 10 includes glass core 12 and glass cladding 14 surrounding the core. Core 12 is made of glass material that is etchable in an appropriate etching solution. Glass cladding 14 is made from glass material which has a softening temperature substantially the same as the glass core. The glass material of cladding 14 is different from that of core 12, however, in that it has a higher lead content, which renders the cladding non-etchable under the same conditions used for etching the core material. Thus, cladding 14 remains after the etching of the glass core. A suitable cladding glass is a lead-type glass, such as Corning Glass 8161.
The optical fibers are formed in the following manner: An etchable glass rod and a cladding tube coaxially surrounding the rod are suspended vertically in a draw machine which incorporates a zone furnace. The temperature of the furnace is elevated to the softening temperature of the glass. The rod and tube fuse together and are drawn into a single fiber 10. Fiber 10 is fed into a traction mechanism in which the speed is adjusted until the desired fiber diameter is achieved. Fiber 10 is then cut into shorter lengths of approximately 18 inches.
Several thousands of the cut lengths of single fiber 10 are then stacked into a graphite mold and heated at a softening temperature of the glass to form hexagonal array 16, as shown in FIG. 2. As shown, each of the cut lengths of fiber 10 has a hexagonal configuration. The hexagonal configuration provides a better stacking arrangement.
The hexagonal array, which is also known as a multi assembly or a bundle, includes several thousand single fibers 10, each having core 12 and cladding 14. Bundle 16 is suspended vertically in a draw machine and drawn to again decrease the fiber diameter, while still maintaining the hexagonal configuration of the individual fibers. Bundle 16 is then cut into shorter lengths of approximately 6 inches.
Several hundred of the cut bundles 16 are packed into a precision inner diameter bore glass tube 22, as shown in FIG. 3. The glass tube has a high lead content and is made of a glass material similar to glass cladding 14 and is, thus, non-etchable by the etching process used to etch glass core 12. The lead glass tube 22 eventually becomes a solid rim border of the microchannel plate.
In order to protect fibers 10 of each bundle 16, during processing to form the microchannel plate, a plurality of support structures are positioned in glass tube 22 to replace those bundles 16 which form the outer layer of the assembly. The support structures may take the form of hexagonal rods of any material having the necessary strength and the capability to fuse with the glass fibers. Each support structure may be a single optical glass fiber 24 having a hexagonal shape and a cross-sectional area approximately as large as that of one of the bundles 16. The single optical glass fiber, however, has a core and a cladding which are both non-etchable. The optical fibers 24, or support rods 24, are illustrated in FIG. 3, as being disposed at the periphery of assembly 30 and surrounding the plurality of bundles 16.
The support rods may be formed from one optical fiber or any number of fibers up to several hundred. The final geometric configuration and outside diameter of one support rod 24 is substantially the same as one bundle 16. The multiple fiber support rods may be formed in a manner similar to that of forming bundle 16.
Each bundle 16 that forms the outermost layer of fibers in tube 22 is replaced by a support rod 24. This is preferably done by positioning one end of a support rod 24 against one end of a bundle 16 and then pushing support rod 24 against bundle 16, until bundle 16 is out of tube 22. The assembly formed when all of the outer bundles 16 have been replaced by support rods 24 is called a boule, and is generally designated as 30 in FIG. 3.
Boule 30 is fused together in a heating process to produce a solid boule of rim glass and fiber optics. The fused boule is then sliced, or diced, into thin cross-sectional plates. The planar end surfaces of the sliced fused boule are ground and polished.
In order to form the microchannels, cores 12 of optical fibers 10 are removed, by etching with dilute hydrochloric acid. After etching the boule, the high lead content glass claddings 14 remains to form microchannels 32, as illustrated in FIG. 4. Also, support rods 24 remain solid and provide a good transition from the solid rim of tube 22 to microchannels 32.
Additional process steps include beveling and polishing of the glass boule. After the plates are etched to remove the core rods, the channels in the boule are metallized and activated.
As described, the current method of manufacturing an MCP includes stacking multiple bundles, and then placing the stacked bundles within a sheath of rim glass. The supporting rods of non-etchable fibers are then used to fill the interstitial space between the bundles of etchable fibers and the rim glass (tube 22) to form a boule. The boule is then sliced at an angle into thin wafers to produce a bias angle. The wafers are then etched, hydrogen fired to form a conduction layer, and metallized to provide electrical contact.
After the boule is sliced into wafers, each wafer is handled individually. A typical size of the wafer is approximately 1 inch diameter. This is much smaller than the wafer size of current semiconductor processing tools and necessitates use of custom fabrication processing tools. Handling each boule wafer individually leads to large amounts of touch labor for a part very sensitive to particle contamination. The yield of these wafers are, therefore, reduced.
The present invention addresses the need for fabricating MCPs using more efficient fabrication methods and for methods that are less subject to contamination and reduced yield.